Device and method for taking spectroscopic polarimetric measurements in the visible and near-infrared ranges

Abstract

A spectroscopic polarimetric system of broad spectral range, includes a light source suitable for emitting an incident light beam over a wavelength range, a polarization state generator (PSG), a polarization state analyzer (PSA), and a detector. The PSG and the PSA have respective elements for modulating the polarization of the light beam. The elements of the PSG for modulating polarization are suitable for generating a sequence of m polarization states with m>4 at each measurement wavelength, the elements of the PSA for modulating polarization are suitable for determining a sequence of n polarization states with n>4 for each measurement wavelength, and the detector elements are suitable for acquiring a sequence of N measurements with 16<N≦n×m at each wavelength to extract therefrom a polarimetric spectroscopic measurement of the Mueller matrix of the sample. An extended spectroscopic polarimetric measurement method is also described.

The invention claimed is: 
     
       1. A spectroscopic polarimetric system for analyzing a sample ( 8 ), the system comprising:
 an excitation portion comprising:
 a light source ( 7 ) suitable for emitting an incident light beam ( 17 ) over a wavelength range; and 
 a polarization state generator (PSG) ( 5 ) comprising:
 a linear polarizer ( 10 ) and means for modulating the polarization of the light beam; 
 
 
 an analyzer portion comprising:
 a polarization state analyzer (PSA) ( 6 ) comprising:
 means for modulating the polarization of the light beam and a linear polarizer ( 10 ′); and 
 
 detector means ( 9 ) for detecting the light beam as a function of wavelength and including a processor unit; 
 
 the system being characterized in that: 
 the means for modulating the polarization of the PSG ( 5 ) comprise three liquid crystal devices ( 1 ,  2 ,  3 ) and voltage control means applied to each of the liquid crystal devices ( 1 ,  2 ,  3 ) suitable for modulating the orientation and/or the delay of the polarization states so as to generate a sequence of  m  polarization states with m>4 at each measurement wavelength; 
 the means for modulating the polarization of the PSA ( 6 ) comprise three liquid crystal devices ( 1 ,  2 ,  3 ) and voltage control means applied to each of the liquid crystal devices ( 1 ,  2 ,  3 ) suitable for modulating the orientation and/or the delay of the polarization states so as to determine a sequence of  n  polarization states with n>4 at each measurement wavelength; and 
 the detector means are suitable for acquiring a sequence of N light intensity measurements where:
   16 <N≦n×m    
 
 
       at each wavelength to extract the Mueller matrix of the sample ( 8 ) therefrom. 
     
     
       2. A polarimetric system according to  claim 1 , characterized in that the excitation portion includes a retractable platform ( 21 ) situated between the PSG ( 5 ) and the sample ( 8 ) and suitable for introducing at least one calibration element in the optical system of the polarimetric system and for withdrawing said calibration element once calibration has been terminated, and the analyzer portion includes a retractable platform ( 21 ′) situated between the PSA ( 6 ) and the sample ( 8 ) and suitable for introducing at least one calibration element in the optical system and for withdrawing said calibration element once calibration has been terminated. 
     
     
       3. A polarimetric system according to  claim 1 , characterized in that the means for modulating the polarization of the PSG are suitable for generating a sequence of m=8 polarization states at each measurement wavelength, the means for modulating polarization of the PSA are suitable for determining a sequence of n=8 polarization states at each measurement wavelength, and the detector means are suitable for acquiring a sequence of N=64 measurements at each wavelength to extract the Mueller matrix of the sample ( 8 ) therefrom. 
     
     
       4. A polarimetric system according to  claim 1 , characterized in that the means for modulating the polarization of the PSG are suitable for generating a sequence of m=6 polarization states at each measurement wavelength, the means for modulating the polarization of the PSA are suitable for determining a sequence of n=6 polarization states at each measurement wavelength, and the detector means are suitable for acquiring a sequence of N=36 measurements at each wavelength to extract the Mueller matrix of the sample ( 8 ) therefrom. 
     
     
       5. A polarimetric system according to  claim 1 , characterized in that the means for modulating the polarization of the PSG comprise three ferroelectric liquid crystal (FLC) devices ( 1 ,  2 ,  3 ) suitable for generating a sequence of m=8 polarization states, and in that the means for modulating the polarization of the PSA comprise three FLC devices ( 11 ,  12 ,  13 ) suitable for determining a sequence of n=8 polarization states. 
     
     
       6. A polarimetric system according to  claim 5 , characterized in that the polarization state generator ( 5 ) includes a delay plate ( 4 ) located between two of the FLCs ( 1 ,  2 ) or ( 3 ,  2 ), and in that the polarization state analyzer ( 6 ) includes a delay plate ( 14 ) located between two FLC devices ( 11 ,  12 ) or ( 12 ,  13 ). 
     
     
       7. A polarimetric system according to  claim 6 , characterized in that the delay plates ( 4 ,  14 ) are achromatic double prisms. 
     
     
       8. A polarimetric system according to  claim 1 , characterized in that the liquid crystal cells ( 1 ,  2 ,  3 ,  11 ,  12 ,  13 ) are nematic liquid crystal cells and in that the polarimetric system includes an electronic control device suitable for modulating the delays of the nematic liquid crystal cells ( 1 ,  2 ,  3 ,  11 ,  12 ,  13 ). 
     
     
       9. A polarimetric system according to  claim 8 , characterized in that the means for modulating the polarization respectively of the PSG and of the PSA comprise respectively three nematic liquid crystal (NLC) devices ( 1 ,  2 ,  3 ) and ( 11 ,  12 ,  13 ), and in that voltage control means are suitable for switching the delay of each NLC device ( 1 ,  2 ,  3 ,  11 ,  12 ,  13 ) in such a manner as to generate a sequence of m=8 polarization states and respectively to determine a sequence of n=8 polarization states. 
     
     
       10. A polarimetric system according to  claim 1 , characterized in that the polarimetric system is optimized for the spectral range 350 nm to 2 μm. 
     
     
       11. A polarimetric system according to  claim 1 , wherein the polarimetric system is an ellipsometer. 
     
     
       12. A polarimetric system according to  claim 1 , wherein the polarimetric system is a Mueller polarimeter for analyzing a sample ( 8 ) from a sequence of N detected light intensity measurements where 16<N≦64. 
     
     
       13. A polarimetric system according to  claim 1 , characterized in that the detector means ( 9 ) comprise an imaging detector adapted to the processor unit to form a polarimetric image of the sample ( 8 ). 
     
     
       14. A method of spectroscopic polarimetric measurement of a sample ( 8 ), the method comprising the following steps:
 illuminating the sample ( 8 ) by means of a polarized incident light beam ( 17 ) emitted by a polarization state generator (PSG) ( 5 ) including a polarizer, said PSG modulating the polarization state of the light beam ( 17 ), said sample ( 8 ) transmitting or reflecting the polarization modulated light beam; 
 detecting the measurement by means of a detector and a detection section comprising a polarization state analyzer (PSA) ( 6 ) and a polarizer, said PSA determining the polarization state of the detected light beam; and 
 processing the detected signals in order to extract a polarimetric measurement of the sample therefrom; 
 the method being characterized in that the polarization states generated by three liquid crystal devices ( 1 ,  2 ,  3 ) are modulated in a sequence of m>4 polarization states, the polarization states analyzed by three liquid crystal devices ( 11 ,  12 ,  13 ) are determined in a sequence of n>4 polarization states, and a sequence of N=n×m measurements. 
 
     
     
       15. A spectroscopic polarimetric measurement method according to  claim 14 , characterized in that a sequence of eight polarization states is generated, a sequence of eight polarization states is analyzed, and a sequence of 64 measurements is acquired at each wavelength. 
     
     
       16. A spectroscopic polarimetric measurement method according to  claim 14 , characterized in that a sequence of six polarization states is generated, a sequence of six polarization states is analyzed, and a sequence of 36 measurements is acquired at each wavelength. 
     
     
       17. A polarimetric system according to  claim 2 , characterized in that the means for modulating the polarization of the PSG are suitable for generating a sequence of m=8 polarization states at each measurement wavelength, the means for modulating polarization of the PSA are suitable for determining a sequence of n=8 polarization states at each measurement wavelength, and the detector means are suitable for acquiring a sequence of N=64 measurements at each wavelength to extract the Mueller matrix of the sample ( 8 ) therefrom. 
     
     
       18. A polarimetric system according to  claim 2 , characterized in that the means for modulating the polarization of the PSG are suitable for generating a sequence of m=6 polarization states at each measurement wavelength, the means for modulating the polarization of the PSA are suitable for determining a sequence of n=6 polarization states at each measurement wavelength, and the detector means are suitable for acquiring a sequence of N=36 measurements at each wavelength to extract the Mueller matrix of the sample ( 8 ) therefrom.